[17.0] Missions To The Comets

v1.4.0 / chapter 17 of 19 / 01 aug 14 / greg goebel / public domain

* The comets, travelers from the far reaches of the solar system, long seemed
mysterious, but now through Earth-based observations and an increasing number
of space probes, their origins and nature are becoming clearer. This chapter
describes comets and the efforts to explore them.

* Comets have been known since antiquity, and in fact their name is derived
from the classic Greek "kometes (hairy object)". They appeared in the night
sky from out of nowhere, growing in brightness and often sprouting a long
"tail" that stretched across the heavens, pointing away from the Sun. Comets
were often regarded as evil omens, since their ghostly appearance and motion
across the otherwise unchanging night sky made them seem like malevolent
intruders. A bright comet preceded the Norman invasion of England in 1066,
with its passage recorded in the Bayeux Tapestry that recorded the events of
the campaign.

Comets were believed to be apparitions in the Earth's atmosphere until 1577,
when the Danish astronomer Tycho Brahe performed precise parallax
measurements of the position of a comet that showed it was at least four
times as far away as the Moon. After Tycho, astronomers acquired better
instruments for observing comets, identifying that they had a small "nucleus"
that produced a large cloudlike "coma" as well as the occasional tail.
Astronomers were also able to realistically plot the trajectories of comets.

In 1682, the British astronomer Edmond Halley used such data and the
gravitational equations provided by his friend Sir Isaac Newton to calculate
the orbit of a great comet that appeared in the sky in that year. Halley
also calculated the orbits of 23 other comets from old observational data,
publishing a catalog of them in 1705. Although the trajectories of these
comets were not known with very great certainty, Halley was able to plot
their orbits as very long and narrow ellipses that looped around the Sun at
one end and curved out into deep space at the other end. His calculations
suggested to him that the 1682 comet had appeared previously in 1607 and
1531. He correctly predicted that "Halley's Comet" would appear again in
1759.

Comets were of great interest to astronomers for centuries, and astronomers
were eager to discover and name them. One of the most famous comet-hunters
was the 18th-century French astronomer Charles Messier, who discovered 21
comets. Ironically, Messier is actually remembered for compiling a list of
"nebulous" objects in the sky in order to help other comet-hunters avoid
confusion from fuzzy objects that looked like comets but weren't. Many
bright nebulas in our Galaxy and bright galaxies are now best known by the
"Messier numbers". Modern professional astronomers generally have other
things to do than worry about comets, and delegate this task to a
well-organized worldwide network of dedicated amateur astronomers, who are
perfectly happy to be given the opportunity to carry on this tradition of
discovery.

* Through the 19th century, more comets were discovered and their orbits
plotted. The orbits ranged in period from a few years to well longer than
human history. Sometimes the orbits of comets seemed to vary somewhat from
those predicted from observations, though this could partly be explained by
the gravitational influence of one of the Outer Planets, particularly
Jupiter.

In addition, many comets seemed to be organized into "groups", as if they had
all split off from a common larger comet and still retained similar orbits.
Periodic "meteor showers" that fell on Earth also often seemed to be
associated with certain comets, as if the comet were trailing fine debris
that then flashed into the atmosphere and disintegrated. Giovanni
Schiaparelli proved that the Perseid meteors, which appear in August, move in
the same orbit as Comet 1862 III. The Leonid meteors, which appear in
November, were also found to follow the orbit of Comet 1866 I.

Even with the development of ever more powerful telescopes, the tiny nucleus
of the comet was never seen as more than a point of light, but late in the
century the development of astronomical spectroscopy began to provide clues
to the nature of comets, showing that they were cold objects made up of dust
and gases. However, astronomers didn't begin to acquire a more detailed
understanding of the nature and origins of comets until the second half of
the twentieth century.

* Astronomers have a formal naming and cataloging scheme for comets,
introduced by the German publication "Astronomische Nachrichten (Astronomical
Reports)" in 1870. When a comet is initially spotted, it is given a
temporary designation consisting of the year in which it is discovered and a
lowercase roman letter, with comets discovered on successive dates arranged
in alphabetical order, for example "1990a", "1990b", "1990c", and so on.

Once the comet orbit has been determined, the comet is given a permanent
designation, defined by the date of the comet's "perihelion", or closest
approach to the Sun. The permanent designation has the year of the
perihelion, followed by a roman number giving the order of the comet's
perihelion passage for that year, for example: "1990 I", "1990 II", and
"1990 III". Of course, the order that results from this resorting may not
match the order of the temporary designations.

Along with the designation, the comet is given a name, usually that of its
discoverer or discoverers. A "P/" is prefixed if the comet is periodic. For
example, Comet Halley is more formally known as "P/Halley", though this
naming is something of an anomaly, since that comet had been observed long
before Halley was born. If the same person discovers more than one comet, an
arabic number is used to distinguish them, for example "Tempel 1 (1867 II)"
and "Tempel 2 (1873 II)". No more than three discoverers are ever listed
together in the name. If there are more discoverers, the comet is simply
given an arbitrary name, such as "Southern Comet (1947 XII)".

Cometary data is recorded in the master "Catalog of Cometary Orbits". It
is published regularly as new information becomes available. The 2005
edition listed 2,991 appearances of 2,241 comets.

* Comets have elliptical orbits, but the size of these orbits varies across
an enormous range. Comets are grouped according to their orbital periods.
Those with periods longer than 200 years are "long-period" comets, which may
have orbital periods in the hundreds of thousands of years or longer. Those
with periods under 200 years are "short-period" comets. The short-period
comets are further split into two subgroups consisting of
"intermediate-period" or "Halley-type" comets with periods from 20 to 200
years, and "Jupiter-period" comets with periods of up to 20 years. In more
detail:

The known long-period comets come from any direction in the sky, and are
just as likely to have retrograde as prograde orbits. When most are
spotted it is for the first time, and in fact some of them apparently have
never made a trip to the inner solar system before, having only recently
(in cosmic terms at least) been disturbed by some event into doing so.
Comet Hyakutake, which visited the inner solar system in 1996, is a good
example of a long-period comet, with a period of 10,000 years. The
spectacular Comet West of 1976 has a period of half a million years.

The known intermediate-period comets tend to follow the "ecliptic", the
plane in which the planets orbit, and most have prograde orbits. The most
famous of the intermediate-period comets is of course Comet Halley, whose
passage has been recorded 30 times, followed by Comet Tempel-Tuttle,
whose passage has been recorded four times.

The known Jupiter-period comets have an average period of seven years, a
perihelion (closest approach to the Sun) of 1.5 times the distance from
Earth to Sun, and low orbital inclinations to the ecliptic. They all have
prograde orbits. The most famous of the short-period comets are Comet
Encke, with a period of 3.3 years, and Comet Tempel 2.

It is possible that a comet could collide with the Earth with disastrous
consequences, though the threat from an Earth-crossing asteroid is greater
since their orbits consistently cross the Earth's orbit. It is far more
likely that comets will collide with Jupiter, which due to its large mass has
much more influence on cometary orbits, trapping them into new orbits or
sometimes ejecting them from the solar system.

As discussed previously, in 1992 Comet Shoemaker-Levy 9 broke apart into 21
large fragments as it ventured into the strong gravitational field of the
planet Jupiter and was trapped into an orbit that terminated on Jupiter
itself. During a week-long bombardment in July 1994, the fragments crashed
into Jupiter's dense atmosphere at speeds of about 210,000 KPH. Upon impact,
the tremendous kinetic energy of the comets was converted into heat through
massive explosions, some resulting in fireballs larger than the earth.

* In deep space, comets are just tiny motes of "sky junk", typically about
ten kilometers across. As the comet approaches the Sun, the tiny "nucleus"
often begins to shed a cloud of gas and dust -- the coma mentioned above.
When the comet gets to within about 1.5 or 2 astronomical units of the Sun,
it may also generate a long "tail" of gas and dust that is popularly regarded
as the trademark of comets. A coma may be about 100,000 kilometers across,
while a spectacular tail may be 100,000,000 kilometers long. However, some
comets generate a feeble coma and tail.

Incidentally, the tail does not trail the direction of the comet's flight.
It is always pointed away from the Sun, and so will follow the comet as it
falls toward the Sun and lead the comet as it moves away from the Sun.

A comet can actually have two distinct tails. The most prominent is the long
"plasma tail", due to solar wind particles stripping atoms of the comet.
This tail is long, bluish, and can feature twists and knots caused by
fluctuations in the solar wind. When the comet gets closer to the Sun, the
sheer pressure of sunlight can blow heavier dust off the nucleus, forming a
wide, yellowish, and featureless "dust tail". Since sunlight streams
directly out of the Sun while the solar wind is dispersed in a spiral, the
two tails may not always point in precisely the same direction. The tails
are very tenuous, and though horror movies have suggested that a passage by
the Earth through the tail of a comet might bring the dead back to life as
flesh-eating zombies, in fact the Earth has passed through the tails of
comets several times with no real effect -- at least so far.

* Early spectroscopic analysis of comets showed that they were cold, shining
only with light reflected from the Sun, and gave hints to their composition.
The coma seemed to be composed of simple compounds of hydrogen, carbon,
nitrogen, and oxygen.

The fact that comets seemed to be related to meteor showers suggested to some
theorists that they were relatively insubstantial objects, little more than a
cosmic "sandbank" lightly glued together by ices. In 1949, the American
astronomer Fred L. Whipple came up with the currently accepted model of
cometary structure, describing a comet as a "dirty snowball" of ice and dust,
with the ices consisting of frozen methane (CH4), ammonia (NH3), and water
(H2O). Whipple reasoned that the nucleus of the comet had to be more
substantial than a sandbank, since "Sun-grazing" comets that passed close by
the Sun often survived intact, though sometimes they split into fragments.

The "dirty snowball" hypothesis not only accounted for spectroscopic
observations of comets, but also helped account for the variability in comet
orbits, which could not be explained completely by the influence of Jupiter
or other planets. Evaporation of the ices as the comet approached the Sun
could create an asymmetrical "jet" that modified its orbital path.

* Not long after Whipple described the structure of comets, other astronomers
provided suggestions as to their origins. In 195O, the Dutch astronomer Jan
Hendrik Oort, following up a suggestion made in 1932 by the Estonian-born
astronomer Ernest J. Opik, proposed that the long-period comets actually
originate from a huge, tenuous, spherical cloud of comets far beyond the
orbit of Pluto, orbiting the sun at tens of thousands of AU. This region has
become known as the "Oort Cloud" in his honor.

Oort believed that stars passing by the solar system could disrupt comets
from their huge, lazy orbits in the Oort Cloud. The comets would either
ejected into space or sent falling into the inner solar system. There, the
giant planets, particularly Jupiter, would modify their orbits to either
throw them out of the solar system forever or trap them as short-period
comets.

The existence of the Oort Cloud is only known by inference from the
calculation of the remote end of the orbits of long-period comets. The
distant comets that reside in the cloud are much too small and far away to be
detected by any modern telescope. Brian G. Marsden and his colleagues
followed up Oort's analysis of the orbits of long-period comets and confirmed
the existence of the Oort cloud.

* Oort based his studies on the orbits of 19 long-period comets, but now
several hundred are now known. Comets entering the region of the planets for
the first time come from an average distance of 44,000 AU, and have periods
of 3.3 million years.

On the average, stars will pass to within a few light years of the Sun once
every million years; within 10,000 AU of the Sun once every 36 million years;
and within 3,000 AU once every 400 million years. It unlikely that any star
has passed within 900 AU of the Sun more than once during the entire history
of the solar system. Most stars are small, dim red dwarfs that couldn't be
seen by the naked eye at such distances, but their passage can send comets
falling into the region of the planets for millions of years, resulting in a
high rate of planetary impacts.

The primordial composition of comets means they have concentrations of
isotopes different from those found on Earth, such as a high concentration of
the rare "helium 3" isotope. A high concentration of helium 3 has been
detected in some sedimentary samples dated to the end of the Eocene epoch, 36
million years ago. This may be due to a period of planetary bombardment that
lasted for two or three million years and led to a moderate phase of
extinctions. In 1.4 million years, the red dwarf star Gliese 710 will pass
through the Oort Cloud at a distance of about 70,000 AU of the Sun. It is
not likely to cause much trouble, increasing the rate of comet infall from
the cloud by only about 50%.

Comets in the Oort Cloud may also be affected by tidal forces caused by the
mass concentrations of the disk of the Milky Way Galaxy, as well as by the
galactic core. In addition, they may be shaken loose by a passing cloud of
cold molecular hydrogen. Such events are rare, occurring two or three times
over a billion years, but since such clouds may be a million times more
massive than the Sun they could have a substantial effect.

* The distant Oort Cloud is huge but tenuous. The number of comets in the
cloud is estimated at about 6 x 10^12. The average mass of a comet is
estimated at about 40 x 10^9 tonnes, which makes the entire Oort Cloud about
40 times more massive than the Earth. This is actually a very low density,
given the vast volume of space occupied by the cloud. On the average, each
comet in the cloud is separated from its neighbors by tens of millions of
kilometers. From this distance, the Sun is no more than a very bright star.
The surface temperature of these comets is no more than about four degrees
above absolute zero.

The comets in the Oort Cloud were almost certainly formed during the original
creation of the solar system. Since they were created at varying distances
from the Sun, they are also likely to have a range of compositions, which has
been observed in long-period comets falling through the inner solar system.

It is likely that other stellar systems have Oort Clouds, and that as with
our own they leak comets into interstellar space. It is also possible that
such an "interstellar" comet could fall into the solar system, with its
interstellar origin revealed by a very high velocity. No such interstellar
comets have been observed in the history of modern astronomy, and nobody's
counting on seeing one any time soon.

* The discovery of the Oort Cloud explained the origins of the long-period
comets, but it left the origins of the short-period comets something of a
mystery. Comets have finite lifetimes. Comet Halley loses about a
ten-thousandth of its mass on every pass around the Sun, meaning it has a
lifetime of well under a million years, a very short time on a cosmic scale.
That means that short-period comets are replenished on a regular basis,
otherwise they would have all disappeared by now.

Astronomers once believed that the short-period comets were originally
long-period comets that were captured by interactions with the planets,
primarily Jupiter. In 1951, the Dutch-American astronomer Gerard P. Kuiper
of the University of Chicago proposed instead that the intermediate-period
comets originated from a belt of such objects beyond the orbit of Neptune and
Pluto, now known in his honor as the "Kuiper Belt". Occasionally the passage
of Neptune would disrupt the orbit of a Kuiper Belt comet, causing it to fall
into the central solar system. Intermediate period comets could then be
trapped by Jupiter or another planet into becoming Jupiter-period comets.

Kuiper believed that the processes that created the planets from the original
stellar disk of dust and gas did not end abruptly at Neptune or Pluto. The
density of matter in the disk was too low beyond those limits to lead to the
creation of a large planet, but it could create a large set of small, icy
bodies. In 1964, a team led by Fred Whipple followed up Kuiper's ideas,
performing an analysis of the orbits of intermediate-period comets, and
concluded that the mass of the Kuiper Belt would be no more than 0.8 Earth
masses if it was at 40 AU from the Sun, or 1.3 Earth masses if it was at 50
AU.

Many astronomers still preferred to believe that the Oort Cloud was the
ultimate source of all comets, and Kuiper died in 1973 without seeing the
acceptance of his theory. However, during the 1970s, Paul C. Joss of the
Massachusetts Institute of Technology performed an analysis that showed the
probability of Jupiter capturing a long-period comet was so low that such a
mechanism could not possibly account for the number of short-period comets
that were actually observed.

In 1977, Charles Kowal also discovered an asteroid of sorts between the
orbits of Jupiter and Uranus. The object, which was named "2060 Chiron", was
several hundred kilometers across and proved to have an unstable orbit,
suggesting that it had come from someplace else and that there might be more
like it from wherever it had come from.

In 1980 Julio Fernandez, then at the Max Planck Institute in Germany,
revisited Kuiper's ideas in a paper that showed the Kuiper Belt would be a
far more likely source of short-term comets than the Oort Cloud. The results
of computer simulations published by Martin Duncan and colleagues at the
University of Toronto in 1988 also cast doubt on the idea that the
short-period comets came from the Oort Cloud. The simulations not only
showed that the probability of capture was low, but that it was very unlikely
that they would be captured into orbits that all lay near the plane of the
ecliptic. This was a weakness of the concept that should have been apparent
from the start, though some astronomers argued that interactions of
high-inclination comets might destroy them more quickly than low-inclination
comets.

Another piece of evidence that pointed to the existence of the Kuiper Belt
was the discovery of "disks" of gas and dust around distant young stars. The
disks were several hundred AU across, much bigger than the orbit of Pluto.
This suggested that the Sun may have had a similar disk in its early days
that eventually coalesced into small objects that we would later see as
comets.

* By this time, observational astronomers were searching for the mysterious
"Kuiper Belt Objects (KBOs)". Everyone knew that such small and distant
objects would be hard to find, but the first was located on 30 August 1992 by
astronomers David Jewitt of the University of Hawaii and Jane X. Luu of
Stanford University. They had been searching for KBOs for five years up to
that time.

The new object was designated "1992 QB1". Since the object was far from the
Sun, it had a very slow orbit, and it wasn't until the end of 1992 that Brian
Marsden determined that it went around the Sun once every 290 years at an
average distance of 44 AU, in a near-circular orbit inclined only 2.2 degrees
to the ecliptic. The large distance, low inclination, and low eccentricity
matched Kuiper's predictions. 1992 QB1 appeared to have a reddish color,
indicating the presence of carbon compounds, as is the case for some comets.
Its diameter was estimated as in the range of 200 to 250 kilometers.

Finding one object was inconclusive, but Jewitt and Luu quickly discovered a
second, which was designated "1993 FW". Observations of its orbit showed it
to have an average orbital radius of slightly less than 44 AU, with low
eccentricity and an orbital inclination of 7.7 degrees. 1993 FW appeared to
be about the same size as 1992 QB1, though it was found in almost the
opposite region of the sky. 1992 QB1 now seemed less like a fluke. Within a
decade of the original discovery, about a thousand KBOs had been located.

The existence of the Kuiper Belt is now generally accepted by astronomers,
and the comet-capture hypothesis has faded away. The discovery of the Kuiper
Belt has led to a change in our understanding of the outer reaches of the
Solar System, a matter discussed in more detail later.

* Comet Halley is the comet most familiar to the public. Although brighter
comets have passed through the neighborhood, Comet Halley is one of the few
bright comets that comes by on an interval short enough to allow a person to
see it twice, once in youth and once in old age.

After Halley's calculation of its orbit in 1682, it appeared again in 1759,
as Halley had predicted, and then in 1835, 1910, and 1986. Since the orbit
of the comet has no particular synchronization to the orbit of the Earth, it
may be closer or nearer to the Earth on each pass around the Sun. The 1986
visitation was a public disappointment, since the comet was relatively far
away during its passage, and it was also not easily seen from northern skies.
This was more than compensated for by the fact that several spacecraft took
the opportunity to inspect Comet Halley during its passage. The spacecraft
included:

The European Space Agency's "Giotto", which flew by the nucleus of the
comet at a distance of less than 600 kilometers and returned high-quality
images.

The Soviet-led "Intercosmos" consortium's "Vega 1" and "Vega 2", which
dropped lander probes on Venus, and then performed flybys of the comet.

The Japanese Institute of Space and Astronomical Science's "Sakigake" and
"Suisei". These were small, simple spacecraft and performed only limited
observations, having been developed by ISAS as a first step in Japanese
planetary missions.

The United States embarrassingly did not send a probe to Halley's Comet,
proposals for doing so having been given the budgetary axe a few years
earlier. NASA did conduct a modest comet mission named the "Interplanetary
Cometary Explorer (ICE)" by diverting a space physics studies probe that had
been launched some years earlier from its orbit between the Earth and Sun.
ICE performed observations of Comet Giacobini-Zinner in September 1985 to at
least show the flag during the Halley activities.

* The ESA Giotto probe was a drum-shaped, spin-stabilized spacecraft derived
from the earlier ESA-GEOS space physics studies spacecraft, with its sides
covered with solar cells, a high-gain antenna on one end, and protected from
coma debris on the other end by an impact shield made of millimeter-thick
aluminum and 12-millimeter-thick Kevlar separated by a gap. Giotto had a
launch weight of about 583 kilograms and was designed to perform ten
different experiments, with an instrument suite including a color camera,
mass spectrometers, particle analyzers, and a magnetometer.

The spacecraft was launched by an Ariane 1 booster from the ESA space center
at Kourou, French Guiana, on 2 July 1985. It performed its flyby of Comet
Halley on 13 March 1986, at a distance of 0.89 AU from the Sun and 0.98 AU
from the Earth, with the closest approach to the comet nucleus measured at
596 kilometers.

Giotto returned spectacular images and other data, but 14 seconds before its
closest approach it was hit by a large dust particle that tipped the
spacecraft so that it was not completely protected by the shield for about a
half hour. Several instruments, including the camera, were disabled.
However, Giotto survived well enough to perform measurements of Comet
Grigg-Skellerup on 10 July 1992, though of course no pictures were returned.
The mission was then formally terminated. Giotto was a major accomplishment
for the ESA, being the agency's first planetary probe and returning excellent
data.

Giotto's observations showed that the nucleus of Halley's comet was indeed a
"dirty snowball", a potato-shaped lump about 15 by 8 kilometers in size and
with a total volume of about 500 cubic kilometers. This was the first time
that a cometary nucleus had ever been seen as anything but a pinpoint of
light. Two different rotation rates were measured, of 2.2 days and 7.3 days,
suggesting it has a complicated multi-axial rotation. Density was estimated
in the low range of 0.1 to 0.8 gram per cubic centimeter. The Soviet Vega
probes estimated the surface temperature as in the range of 300 to 400
degrees Kelvin.

It was a very dirty snowball. The nucleus was extraordinarily black,
with an albedo of no more than about 4%, about as black as velvet, making it
one of the darkest objects in the solar system. The "dirt" was provided by
various sooty carbon-silicate compounds. About ten percent of the comet's
surface showed activity through the surface crust, with circular features of
varying sizes that resembled volcanic vents, with gases vaporizing away under
the influence of the Sun's warming. About 80% of the vaporizing gas was CO2,
followed by CO (about 10%), CO2 (about 4%), and then small quantities of
other molecules, such as methane (CH4), ammonia (NH3), and carbon disulfide
(CS2). Of course, none of the Comet Halley probes had any ability to
determine the composition of the nucleus under the crust.

* The two Soviet Vega probes the ultimate evolution of the Venera series of
Venus probes. Each consisted of a large flyby spacecraft that carried a
Venus lander probe and an ingenious Venusian "weather balloon", discussed
earlier. The flyby spacecraft carried a instrument suite including
narrow-angle and wide-angle cameras, a spectrometer, a magnetometer, and
various plasma probes.

Vega 1 was launched on 15 December 1984 by a Proton booster. It dropped its
Venus probe and balloon on 11 June 1985, and performed a flyby of Comet
Halley on 6 March 1986 at a closest approach of 10,000 kilometers. Vega 2
was launched on 21 December 1984. It dropped its probe and balloon on 16
June 1985, and performed its flyby of Comet Halley on 9 March 1986, at a
closest approach of 3,000 kilometers. Flyby velocities were about 78
kilometers per second. The Vega flyby spacecraft were reasonably
sophisticated, but they had only light impact shielding and so were not able
to obtain the same close-up observations and measurements as Giotto.

* The two Japanese probes were almost identical to each other, differing only
in payload. They both were spin-stabilized, drum-shaped, ringed with solar
cells, and mounted a small high-gain antenna on top. Each weighed a little
less than 140 kilograms and was launched by an ISAS M-3SII booster.

Sakigake, whose name means "Pioneer" in Japanese, was sent into space from
the ISAS launch center at Kagoshima, Japan, on 1 August 1985. It carried
three instruments to study plasmas, the solar wind, and interplanetary
magnetic fields, or in other words was basically a simple space physics
studies probe. Its closest approach to Comet Halley was on 11 March 1986, at
a distance of about 7 million kilometers.

Suisei, which is Japanese for "Comet", was also known as "Planet A" before
launch. It was sent into space from Kagoshima on 18 August 1985. The
payload consisted of an ultraviolet imager and a solar wind instrument.
Suisei performed its closest approach to Comet Halley on 8 March 1986, at a
distance of 151,000 kilometers. ISAS had wanted to fly Suisei by two other
comets, with the encounters in the late 1980s, but depletion of the
spacecraft's fuel supply made this impossible.

* The ICE probe was originally sent into space as the "International
Sun-Earth Explorer 3 (ISEE-3)" or "Explorer 59" on 12 August 1978 on a Delta
booster. It was part of a constellation of three ISEE spacecraft carrying
complementary payloads to investigate the solar wind and the space
environment.

ISEE-3 was a simple spin-stabilized drum ringed with solar cells, sprouting
long whip antennas, and weighing 390 kilograms at launch. It was originally
placed in a wide elliptical "halo" orbit around the "L1 libration point", the
location of the gravitational balance point of the Sun and Earth. It was the
first spacecraft to ever be placed in a halo orbit.

A NASA JPL engineer named Robert Farquhar managed to come up with an
ingenious scheme to compensate for the cancellation of the US Comet Halley
probe, by designing a trajectory to allow it to perform observations of Comet
Giacobini-Zinner. The spacecraft was boosted out of its halo orbit on 10
June 1982, and then made several passes around the Moon for gravity assist
that sent it on its way to the comet.

The spacecraft was then renamed ICE. It passed through the plasma tail of
Comet Giacobini-Zinner on 11 September 1985. It had no imager but it was
able to perform measurements of particles and fields in the tail. ICE made
remote measurements of Comet Halley in March 1986, and then returned
measurements of the space environment from its orbit around the Sun until the
mission was terminated in 1997. Although ICE was the first spacecraft to
inspect a comet, as a comet probe it was too limited to be regarded as
particularly memorable. Its major distinction was its pioneering use of a
series of gravity-assist trajectories of unprecedented sophistication, a
spaceflight technology that has been refined and put to use since that time
by Farquhar and others.

NASA effectively abandoned ISEE-3 / ICE in 1997. There was an attempt in
2014 by a private group, with cooperation from NASA, to "reboot" the
spacecraft -- but though communications were re-established, the thruster
system was no longer in good enough shape for maneuvering. The most that
could be done was perform passive observations.

* After the 1986 Halley missions, no further probes inspected comets to the
end of the century. In the mid-1980s, NASA began work on an ambitious probe
named "Comet Rendezvous Asteroid Flyby (CRAF)", part of the "Mariner Mark II"
series. CRAF proved too ambitious and was canceled, but the arrival of the
new millennium brought along with it an energetic second wave of comet
exploration activities.

Following the Halley probes, the next close-up observations of a comet were
obtained in 2001, almost by a lucky accident. The NASA JPL "Deep Space 1
(DS1)" probe had been launched by a Delta booster on 24 October 1998,
primarily as a technology-demonstration system, though it was to observe
asteroids and possibly comets as part of the flight. DS1 weighed 490
kilograms, was 2.5 meters long, and 1.7 meters wide. It had twin solar
arrays that extended to a span of 11.8 meters. The most significant new
technology tested on DS1 was a solar-electric propulsion system, but it also
tested advanced solar arrays, autonomous mission software, a new
communications system, a compact plasma analyzer, and a miniaturized combined
camera and imaging spectrometer,

DS1 was originally planned to fly by asteroid 3352 McAuliffe and by comet
West-Kohoutek-Ikemura, but in fact neither of these specific objectives were
achieved. On 28 July 1999, DS1 did perform a flyby of the asteroid 9969
Braille, but due to a mission planning screwup, when the images were
downloaded over the next few days, nothing was visible except empty space.
The camera system had been pointed incorrectly. Some belated long-range
photographs were taken, but they showed little more than a fuzzy blob. The
DS1 website trumpeted the "successful" flyby and proudly displayed the fuzzy
little pictures; even more foolishly, the website failed to mention any of
the problems.

However, the probe and the mission team more than redeemed themselves later.
In September 1999, the mission was extended to December 2001 and was given
additional funding. The spacecraft was redirected towards comet Borrelly,
performing its closest approach to the comet at 22,000 kilometers on 22
September 2001. The images returned of the comet's nucleus were excellent,
with a best resolution of 45 meters, superior to the images returned by
Giotto of the nucleus of comet Halley. The comet nucleus was shaped roughly
like a bowling pin, eight kilometers long and four kilometers in diameter at
its widest. It was black as soot, as had been the case with the nucleus of
comet Halley, but featured an interesting mottled pattern. There were
several relatively bright jets of material being emitted from fractures in
the nucleus, detailed in long-exposure images taken by the probe.

One of the mission scientists, Larry Soderblom of the US Geological Survey --
who as mentioned earlier was also a Voyager II project scientist -- was
wildly enthusiastic about the take: "It's mind boggling and stupendous.
These pictures have told us that comet nuclei are far more complex than we
ever imagined. They have rugged terrain, smooth rolling plains, deep
fractures and very, very dark material." Nobody had expected DS1 to do so
well, since it was on its last legs. The mission was terminated in December
2001.

* The NASA "Stardust" probe, another Discovery spacecraft, was launched by a
Delta II booster from Cape Canaveral on 7 February 1999 to rendezvous with
comet Wild 2 (pronounced "Vilt 2") and return samples from its coma and tail
in a reentry capsule. It was the first US planetary sample-return mission
since the Apollo missions to the Moon in the late 1960s and early 1970s; the
first US robotic planetary sample-return mission; and the first planetary
sample-return mission to anyplace but the Moon.

The project was formally initiated in 1995. Launch weight of the
refrigerator-sized Stardust probe was 385 kilograms. The spacecraft was
derived from the "SpaceProbe" deep space bus design developed by Lockheed
Martin Astronautics. Power was provided by two fixed solar arrays that
generated a minimum of 170 watts during the duration of the mission.

After its launch in early 1999, Stardust took an indirect path to comet Wild
2, having taken an orbit around the Sun and performed an Earth flyby in
January 2001 for a gravity assist to set itself up for the rendezvous. It
flew by the nucleus of Wild 2 on 2 January 2004 at a distance of 1.9 AU from
the Sun. Closest approach to the comet nucleus, which was about five
kilometers across, was about 230 kilometers, with a relative speed of 6.1
kilometers per second, only a tenth of the speed of Giotto's flight past
Comet Halley in 1986.

Stardust was protected from impacts by particles up to a centimeter in size
by a set of barriers known as "Whipple shields", in honor of Fred Whipple.
72 pictures were taken using a camera peering out through a periscope to
protect it from particle impacts, with the images having a best resolution of
about 40 meters. Images at closest approach were all grayscale, since the
flyby was rapid and there was no time to build color images using a color
wheel filter. The images showed jets of gas and dusty pouring out of the
nucleus.

Stardust also performed observations of coma particle composition with a mass
spectrometer, and most importantly obtained samples of comet particles with
its sample collection system. The samples were collected in a glass-foam
"aerogel" matrix that was pivoted above the shields on a "tennis racquet"
paddle to trap coma particles. The paddle was 36 centimeters across and had
132 aerogel cells on each side, the reverse side having been used to collect
samples from interplanetary space on the way to the comet. The sample-return
capsule was not vacuum-sealed, since it would have had to be much more rugged
and heavier to withstand atmospheric pressure if it were.

Stardust made a third loop around the Sun and returned to Earth on a flyby
approach to drop its sample return capsule. The capsule was designed to
admit air through filtered vents to ensure that pressure remained in
equilibrium during the descent, with the capsule purged with nitrogen after
recovery. There was little concern that the capsule might return any
dangerous organisms from the comet: life is unlikely on such a frozen
snowball, and in any case hundreds of thousands of tons of cometary material
fall into Earth's atmosphere every year.

The capsule landed by parachute in the deserts of Utah on 15 January 2006.
It reentered over the Pacific off the California coast and left a blazing
streak over Nevada, being observed by a NASA DC-8 jetliner fitted with
spectroscopic gear to characterize the behavior of the capsule's heat shield.
The mission was challenging, but in the end it proved an outstanding success.

Comet Wild-2 was an attractive target for study, because until recently it
was a long-period comet and didn't get any closer to the Sun than the orbit
of Jupiter. In 1974, the comet flew close to Jupiter and was pulled into a
new orbit that brought it in closer, roughly near the orbit of Mars. The
comet was spotted by German astronomer Paul Wild in 1978. Wild-2 is a
relatively pristine comet but also comparatively accessible. The Stardust
probe was then diverted for a second comet flyby, as discussed below.

* Stardust was followed by another Discovery-series comet probe, the "Comet
Nucleus Tour (CONTOUR)", a mission to perform several comet flybys. It was
launched from Cape Canaveral on 3 July 2002 by a Delta 2 booster, and placed
in a high elliptical Earth orbit for checkout preliminary to injection on an
interplanetary trajectory.

CONTOUR was a relatively small and simple spacecraft, shaped like an
octagonal box, ringed with solar cells, and with a launch mass of 328
kilograms. Payload included a spectroscopic imager, a wide-angle camera, a
dust analyzer, and a mass spectrometer. A dust shield made of Nextel and
Kevlar was to protect the spacecraft from dust impacts. The mission was
under the direction of Cornell University, working with NASA, and the probe
itself was built by the Applied Physics Laboratory at Johns Hopkins
University.

CONTOUR was to visit several comets, beginning with a flyby of comet Encke in
November 2003, and then using several Earth flybys to boost it to at least
one more comet. Unfortunately, on 15 August 2002 the probe performed its
engine burn to leave Earth orbit; contact was lost immediately and it
appeared the probe had broken up. No replacement was planned.

* In 1999, NASA approved the development of a third Discovery comet probe,
named "Deep Impact", which was launched by a Delta II 7925 booster from Cape
Canaveral on 12 January 2005, for a flyby of comet Tempel 1 on 4 July 2005.
No particularly complicated series of gravity assists and flybys was required
for the flight; basically, Tempel 1's orbit curves relatively closely around
that of the Earth, and the probe simply intercepted the comet as it neared
its closest approach to our planet. Tempel 1 is about 5 kilometers in
diameter has a period of 5.5 years, and was active at the time of the
rendezvous, when it was about as close to the Sun as it gets.

Launch weight of the probe was 1,010 kilograms, including a 370 kilogram
copper-tipped slug, about the size of a washing machine, that was to be fired
into the comet to impact at almost 36,000 KPH and blast open a deep crater,
allowing the space probe to inspect the interior of the comet. Copper was
used because it was known not to exist in comets and so could be sorted out
in instrument readings from the rest of the ejecta.

The main spacecraft carried a "High-Resolution Imager (HRI)" and a "Medium
Resolution Imager (MRI)". Both consisted of a CCD camera with a filter
wheel; the HRI also includes an infrared spectrometer. The narrow-angle HRI
was used for the detailed observations of the encounter, while the wide-angle
MRI was used for navigation and to provide context images for HRI shots. The
spacecraft was protected from cometary debris by a Whipple shield. The
impactor carried a "Impactor Targeting Sensor (ITS)" to allow the main probe
to observe the comet in detail right up to impact as it flew by at a minimum
approach of about 500 kilometers. The ITS was identical to the MRI but
didn't have a filter wheel.

The rendezvous took place on schedule and the impactor was fired into the
comet, generating much more spectacular "Fourth of July" fireworks than
expected, producing a blast of material from the body of the comet. Mission
scientists took pains to reassure reporters that the impact wouldn't redirect
Tempel 1 so that it might hit Earth.

The impact was visible from ground-based telescopes, and was observed by
several NASA space observatories, Hubble Space Telescope, the Spitzer SIRTF
infrared space telescope, the Chandra X-ray observatory, and even the
Submillimeter Wave Astronomy Satellite, which had been inactive for almost a
year but was specially revived for the occasion. The ESA observed the impact
with the XMM Newton X ray observatory and the Rosetta probe, discussed below.
The Hubble observed the comet increasing in brightness by a factor of six.
Analysis of the "explosion" showed the comet to be covered by fine, powdery
material. In fact, the gusher of powdery material was so dense that it was
difficult to make out any details of the impact crater.

Further analysis showed that the comet was a somewhat insubstantial object,
about half as dense as water overall and full of voids, with a fluffy
structure that is even more weakly packed than a bank of powder snow --
really dirty powder snow, with some commenting that it seemed less like a
"dirty snowball" than a "snowy dirtball". The structure doesn't conduct heat
well, so even when the comet comes relatively close to the Sun, the core
still remains cool. Despite its lack of substance -- some have called it
little more than a "dust bunny" -- the surface of Tempel 1 actually had
distinctive geological features. The comet nucleus also showed some evidence
of layering.

Following the encounter the Deep Impact science probe seemed to be in
excellent condition, so on 20 July it was commanded to perform a course
correction for an Earth flyby on 31 December 2007, which could be used to
redirect the spacecraft to another comet or an asteroid. Following some
consideration of options, the mission was refocused on two tasks -- a "Deep
Impact Extended Observation (DIXI)" to fly another comet, and an "Extrasolar
Planet Observation & Characterization (EPOch)" survey. The mission was then
renamed "EPOXI" as a summary.

The original secondary comet target was short-period comet Boethin, but that
had to be scratched when astronomers couldn't find the comet any longer, the
body having apparently broken up. The actual target selected was comet
Hartley 2. Deep Impact performed four more Earth gravity-assist flybys to
set up the proper trajectory, with encounters on 29 December 2008; a
"distant" flyby on 29 June 2009; another "distant" flyby on 28 December 2009;
and final flyby on 27 June 2010. The flyby of Hartley 2 was performed on 4
November 2010, with a closest approach of about 700 kilometers. Excellent
images were obtained.

The extrasolar planet work included telescope observations of star systems
known to have planets, in hopes of acquiring details of these bodies; and
long-range observations of Earth to help "calibrate" observations of
earthlike bodies elsewhere. Deep Impact was sent on to asteroid
2002GT, with flyby scheduled for January 2020, but the odds of the spacecraft
surviving so far beyond its design life were not good; contact was lost in
August 2013, and the mission was ended.

In another interesting exercise in recycling, a plan was devised to send the
Stardust comet sample-return probe to Tempel 1, with the spacecraft
inspecting the damage inflicted by Deep Impact. The extended mission was
designated "Stardust-NEXT" for "New EXploration of Tempel". The probe
performed the flyby on 14 February 2011. Stardust was then permanently
shut down in March after a few final tests.

* The European Space Agency "Rosetta" comet probe was launched from the ESA
Kourou space center by an Ariane 5G+ booster on 2 March 2004. Rosetta was
designed to rendezvous with a comet and drop a lander named "Philae" to the
comet's surface. The probe was named after the "Rosetta Stone", a tablet
found by the French in Egypt that led to the deciphering of Egyptian
hieroglyphics. Mission scientists felt that analysis of a comet would
similarly provide clues to help decipher the origins of the solar system.
The name "Philae" referred to the obelisk whose markings were matched to
those of the Rosetta stone to help decipher the hieroglyphics.

Rosetta had a launch mass of 3,040 kilograms, including the 100 kilogram
Philae lander. Rosetta was built around an aluminum box with a volume of
about 12 cubic meters. The science payload was fitted in the upper part of
the box, while the flight subsystems -- computer, communications gear, and
propulsion system -- were fitted in the lower part. The propulsion system
consisted of 24 ten-newton thrusters. There was a steerable high-gain
antenna on one side of the box, and the Philae lander on the other.

Rosetta had large steerable solar arrays with a span of about 30 meters and
an area of 64 square meters to allow it to operate in the depths of space
beyond the asteroid belt. These were the largest solar panels carried by a
European spacecraft to that time. The science payload took up a fifth of the
launch weight; in contrast, more than half the launch weight was fuel.
Rosetta itself carried ten experiments, including several cameras and
microwave, infrared, visible, and ultraviolet spectrometers. Instruments
were provided by the US as well as European nations. The spacecraft bus was
built by Astrium, with the spacecraft tested and integrated by Alenia Spazio.

Philae carried 11 experiments totaling 25 kilograms, including a tiny
panoramic camera and a drill capable of boring 30 centimeters into the comet.
Originally, the Rosetta mission was to carry two landers, one built by a
collaboration of the French and Americans named "Champollion", and other
built by the German DLR space agency, originally named "Roland" for "Rosetta
Lander". The US pulled out of the Champollion project, but the Germans went
on to complete Roland, which was renamed Philae not long before launch.

Rosetta's flight plan was elaborate. An Earth flyby was performed on 4 March
2005 for a gravity assist, with the probe imaging and performing other
observations of the Earth and Moon as a test of its instrument suite. The
probe flew by Mars in February 2007 for a second assist, followed by an Earth
flyby in November 2007 and a second Earth flyby in November 2009. The
spacecraft's path took it by asteroid Steins, a small body a few kilometers
in diameter, on 5 September 2008. The probe then flew by asteroid Lutetia, a
larger body about 100 kilometers in diameter, on 10 July 2010.

In mid-2011, Rosetta went into a hibernation mode for two-and-a-half years.
It was revived in early 2014 to perform maneuvers to put it into orbit
around Comet 67P / Churyumov-Gerasimenko by the end of summer of that year.

Churyumov-Gerasimenko was discovered in 1969 and is believed to be three to
five kilometers in diameter. Observations were performed by the NASA Hubble
Space Telescope in 2003 to determine the comet's suitability as a target. At
that time of the rendezvous, the comet will be roughly as far from the Sun as
the orbit of Jupiter. The probe will then perform a long sequence of
observations, with one of the items on the agenda being the selection of a
landing spot for the probe's Philae lander.

The three-legged Philae lander is to descend to the surface of the comet in
November 2014, with its three legs fitted with ice pitons to ensure they get
a grip. Philae will touch down at a velocity of about a meter per second
(walking speed), with the probe firing a harpoon into the asteroid's surface
to keep it from floating off. It will take pictures and perform chemical
analysis for a week or more, with the results relayed back to Rosetta. After
the discovery of just how insubstantial the nucleus of Tempel 1 is, there are
concerns that Philae will simply disappear into the surface of the nucleus.

Rosetta will observe the comet from a distance of about 25 kilometers. It
will watch changes in the comet as it approaches the Sun through the rest of
2014 and up to the end of the mission in late 2015, though of course mission
scientists will extend the observations if possible. Early observations were
performed from the beginning of summer of 2014 and revealed the comet was a
contact binary, with a double nucleus.

Rosetta was originally to be launched in January 2003 to rendezvous with
comet 46P/Wirtanen in 2012. However, after technical difficulties with the
Ariane 5 made meeting the launch window problematic, mission planners decided
to postpone the launch. After considering candidates for new target comets,
the mission team selected Comet Churyumov-Gerasimenko. The mission
engineering team had to modify Philae's landing gear since
Churyumov-Gerasimenko is about four times more massive than Wirtanen,
resulting in higher gravity and a harder touchdown.